What you’ll get here: a beginner-friendly, detailed walk-through of the fuel-line system on Hino W04D / W04C-T / W04C-TI marine engines, why fuel-line repairs are needed, what every component is and does, how the system works, how to replace or repair fuel lines safely, how to bleed the system afterward, and common failure modes and fixes. No fluff.

Safety first
- Work in a well-ventilated area. Diesel vapors and fuel are hazardous.
- Stop the engine and remove ignition/starting power (disconnect battery) before touching fuel lines.
- Wear gloves, eye protection, and keep a fire extinguisher rated for flammable liquids handy. No smoking, no sparks.
- Contain fuel runoff with pans and rags — dispose of used fuel and filters per local rules.
- Consult the OEM workshop manual for torque specs and model-specific procedures.

Quick analogy
- Think of the fuel system as the engine’s circulatory system: fuel tank = reservoir, lift pump = heart that sends fuel up, fuel lines = arteries, filters = kidneys, injectors = capillaries delivering fuel to tissues (combustion). Air in the lines is like an air embolism — it interrupts flow and causes malfunction.

Main components (detailed descriptions)
- Fuel tank: stores diesel. On boats usually vented, may have water drain. Corrosion or contamination here causes many fuel problems.
- Tank shut-off valve (if fitted): stops fuel flow; used for isolation. Check it closes and seals.
- Suction/strainer (tank pickup screen): coarse filter in tank pickup to stop large debris from entering lines.
- Low-pressure hoses (rubber/CR, reinforced): carry fuel from tank to primary filter and pump. Must be diesel-rated and marine-rated (crush-resistant, oil/fuel resistant). Hoses are usually secured by clamps or crimp fittings.
- Primary filter / water separator: removes water and large particulates. Often has a clear bowl with a drain petcock and a manual primer connection. Water collects in the bowl and is drained regularly.
- Manual primer (hand pump): small lever pump used to prime/bleed air and move fuel when lines are opened. Typically between tank and filter or filter and pump.
- Transfer/lift pump (in-tank or inline): low-pressure pump that supplies the injection pump. May be mechanical or electric. It provides a constant supply and slight pressure to avoid cavitation.
- Secondary filter (if fitted): finer media before the injection pump.
- Injection pump (mechanical/rotary/plunger type): pressurizes fuel to injection pressure and times fuel metering. High importance — do not loosen high-pressure fittings with the engine running.
- High-pressure metal injection lines: small-diameter steel tubes from pump to each injector. These take very high pressure and use flare or banjo fittings with crush washers. They must be installed in the correct order and orientation.
- Injectors (nozzles): atomize diesel at high pressure into the cylinder. They have a nozzle body, sealing washer, and high-pressure connection.
- Return line(s): route unused fuel back to tank or to a return header; they relieve excess pressure and provide circulation. Return lines are low-pressure but must be leak-free.
- Banjo bolts and sealing washers/crush washers: special bolts used on some pump/injector connections; use new copper crush washers when reassembling.
- Compression fittings and ferrules (for some low-pressure lines): require proper crimping or tightening to seal.
- Clamps (worm-drive or jubilee; or constant-tension clamps): secure rubber hoses. Use fuel-grade clamps and good practice — not hose clips that can cut soft hoses.

Theory / how the system works (simplified flow)
1. Fuel leaves the tank through the pickup, passes a strainer, and flows through a low-pressure hose to the primary filter/water separator.
2. The manual primer and/or lift pump removes air and pushes fuel to the lift/transfer pump.
3. The lift pump maintains supply to the injection pump at low pressure (a few psi), preventing cavitation.
4. The injection pump pressurizes fuel (hundreds to thousands of psi depending on injection system) and times the pulses to each injector.
5. High-pressure steel lines deliver pressurized fuel to injector nozzles which spray into the cylinder.
6. Excess fuel or leakage returns by return lines to the tank or a return manifold. Air pockets trapped in this loop cause starting/running problems.

Why repair/replace fuel lines
- Leaks: visible fuel leaks are fire hazards, cause loss of fuel pressure, and allow air into the system.
- Aging hoses: softening, cracking, porous swell from age/contamination lets air in or leaks out.
- Blockage or collapse: clogged hoses or collapse under vacuum cause fuel starvation.
- Corrosion/pitting on metal lines or fittings: causes leaks at high pressure.
- Poor fittings/crush washers: old washers no longer seal, connectors loosened previously.
- Water contamination: water leads to corrosion, filter overload, injector damage.
- Vibration/chafing damage: lines rubbing on engine parts will wear through.
Repair is needed to restore proper pressure/flow, prevent air ingress, and eliminate fire hazards.

Tools and parts you’ll need
- Diesel-grade replacement hoses (low-pressure), correct inner diameter and pressure rating; marine-rated if possible.
- Preformed high-pressure lines or OEM replacement injector lines for W04 engines (if a high-pressure steel line is damaged you must replace it with correct spec).
- New crush washers / copper sealing washers for banjo bolts/fittings.
- Fuel filters (primary and secondary) and water separator element.
- Hand primer pump (if not already fitted).
- Spanners including flare-nut / line wrenches of the correct sizes, adjustable wrench, socket set.
- Screwdrivers, hose clamp pliers, needle-nose pliers.
- Clean containers, rags, drip pan.
- Torque wrench and workshop manual for torque specs (very important for injection pump/injector fittings).
- Safety equipment (gloves, goggles, absorbent pads).

Step-by-step — inspecting, removing and replacing fuel lines (beginner-friendly, generic)
Note: Always follow the model-specific marine workshop manual for exact line routing and torque values. Below is a safe general approach.

1. Prepare and isolate
- Park/secure vessel on hard stand or dock safely.
- Ensure engine is off, key removed, battery disconnected.
- Close any tank shut-off valve if fitted.
- Place drip pans under connections you’ll open.

2. Relieve low-pressure system
- Use the manual primer to pull fuel away from the area you’ll open. Open the primary filter drain to remove fuel from filter bowl if necessary.
- For safety, open the primary filter bleed screw (if present) and operate primer to reduce pressure and drain fuel into your catch container.

3. Identify line types & take notes
- Identify which lines are low-pressure rubber and which are high-pressure steel. Take photos and mark orientation/lengths. High-pressure injector lines must be reinstalled exactly the same way.

4. Replace low-pressure rubber hoses
- Loosen clamps and remove hose. Cut off mounting tabs or trim if needed.
- Inspect hose ends and fittings for corrosion. Replace hose; slide clamps on before fitting.
- Use only diesel-rated hose, same ID and appropriate length with slack for vibration.
- Re-secure clamps: place evenly and tighten until snug but not deforming hose. For marine use constant-tension or ear/crimp clamps if recommended.

5. Replace metal injection/HP lines or fittings
- Clean area to prevent dirt into pump or injector.
- Use a flare-nut wrench on fittings to avoid rounding. Remove the high-pressure line at the injector or pump side (one end) first, then the other.
- Replace any damaged lines or fittings. Replace crush washers/copper seals.
- Reinstall metal lines in the exact same routing and orientation. Hand-tighten, then torque to OEM spec (do not guess). Over-tightening can damage fittings; under-tightening leaks.

6. Replace banjo bolts and washers
- Always fit new crush washers on banjo bolts and torque to spec. These provide the seal.

7. Connect and check clamps/fittings
- Make sure hoses aren’t rubbing metal edges and have proper supports/clips. Add protective loom or ties where lines route near moving parts or hot surfaces.

8. Bleed the system (important)
- Open the bleed screw on the primary filter/water separator.
- Operate the manual primer repeatedly until a steady stream of fuel — free of air bubbles — comes from the bleed screw.
- Close the bleed screw. Repeat at any other bleed points shown in manual (some engines have injector bleed screws).
- Reconnect battery, prime any electric lift pump if fitted, then crank engine. If it stalls or runs roughly, re-open the filter bleed and pump until no air bubbles are present.
- After running for a few minutes, check every fitting and hose for leaks under operating condition.

Bleeding tips (practical)
- Visual: you want continuous fuel flow with no bubbles from the bleed screw.
- If air keeps entering, trace upstream — a loose hose clamp, cracked hose, or leaking pump/injection pump seal may be sucking air on suction side.
- If engine starts but dies, wanders, or lacks power: likely still air trapped.

What can go wrong (common faults and fixes)
- Persistent air ingress:
- Causes: cracked suction hose, loose clamps, damaged tank pickup, bad primer diaphragm.
- Fix: inspect suction lines, replace suspect hoses, ensure clamp integrity, check tank pickup filter screen.

- Leaking high-pressure fitting:
- Cause: reused crush washers, over/under-torqued flare nuts, damaged flare surfaces.
- Fix: replace washers, clean mating surfaces, torque to spec.

- Collapsed suction hose:
- Cause: hose not reinforced or age, vacuum from pump.
- Fix: replace with reinforced hose, check for obstructions, ensure pump is functioning.

- Fuel contamination/water:
- Cause: water in tank, leaking deck fills, off-spec filtering.
- Fix: drain tank, replace fuel filters and separators, consider polishing fuel and adding anti-water measures.

- Clogged filter:
- Symptoms: hard starting, rough idle, power loss.
- Fix: replace filter(s) and bleed system.

- Vibration chafe leading to rupture:
- Cause: poor routing and missing clamps.
- Fix: re-route, use clamp brackets and protective sleeves.

- Damaged injector line flare/nut:
- Cause: wrong tools (regular pliers), rounded nuts.
- Fix: replace line or nut, always use correct flare-nut wrench.

- Over-tightening injection fittings:
- Cause: trying to stop a leak with brute force.
- Fix: replace sealing surfaces/washer and torque correctly.

Inspection checklist after repair
- No visible leaks at static and running conditions.
- Engine runs smoothly after bleeding; no sudden loss of power.
- Fuel flow to filter and from return seen (if transparent sections present).
- Hoses secured away from hot/exhaust surfaces and moving parts.
- Drain water from primary filter and check bowl over next few hours running.

Final notes / best practices
- Use OEM parts for high-pressure lines and injection components where possible.
- Always replace crush washers and sealing elements when you open high-pressure fittings.
- Keep a spare primary filter, a set of crush washers, and a small tube of approved thread lubricant/sealant (only where specified by manual).
- Log the repair and date — fuel system maintenance schedule matters more in marine use because of moisture.

This gives you the components, why repairs are needed, how the system functions, a safe replacement/bleed approach, and what can go wrong. Follow model-specific torque values and procedures from the Hino marine workshop manual for critical torque and timing steps.
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